Fix mpmcq use-after-free bug #2722

Job	Run time
Lint bash, docker, markdown, and yaml	1m 14s
Validate GNU Docker image builds	41s
Validate musl Docker image builds	20s
Validate Windows Docker image builds	1m 49s
riscv64 Linux glibc	1h 15m 43s
Validate GNU Docker release image builds	27s
Validate musl Docker release image builds	25s
Validate Windows Docker release image builds	1m 9s
Verify CHANGELOG is valid	8s
x86-64 Linux glibc	20m 53s
x86-64 Linux musl	22m 17s
aarch64 Linux glibc	21m 30s
arm64 Apple Darwin	18m 20s
armhf Linux glibc	1h 21m 15s
arm Linux glibc	1h 25m 7s
x86-64 Apple Darwin	35m 42s
x86-64 Windows MSVC	39m 15s
use dtrace	20m 50s
use pool_memalign	25m 54s
use runtimestats	21m 29s
	7h 54m 28s

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